1. Field of the Invention
The present invention relates to an optical sensor apparatus, and relates more specifically to an intensity-modulated optical sensor apparatus (hereinafter simply "optical sensor") for measuring such physical quantities as current and voltage based on intensity modulation of light. The present invention further relates to a signal processing circuit used in the aforementioned optical sensor apparatus.
2. Description of the Background Art
An optical sensor can be used for measuring current, voltage, and other physical properties based on intensity modulation of light using, for example, the Faraday effect or Pockels effect. Equipment typical of such optical sensors is taught in Japanese Unexamined Patent Publication (kokai) S62-150170 (1987-150170). Such optical sensors are commonly used in the power generating industry, particularly as current-voltage measurement equipment and fault detection equipment on power transmission lines, because of their high withstand voltage and high insulation properties. Application in general purpose measuring instruments has also been considered with the development of high performance, low cost optical sensors, but this requires the development of specialized optical sensor signal processing circuits.
FIG. 3 is a block diagram of an exemplary optical current sensor apparatus according to the related art. As shown in FIG. 3, this optical current sensor apparatus comprises an optical current sensor 30 and an optical current sensor signal processing circuit (hereinafter referred to as signal processing circuit) 31 connected by means of optical fibers 33 and optical connectors 32. The signal processing circuit 31 comprises an optical-electrical conversion circuit 311, subtraction circuit 312, filter 313, LED driver 314, and LED 315.
The optical current sensor 30 contains a magneto-optical crystal (not shown in the figure), detects a change in (the field strength of) the current to be measured, and intensity modulates light according to the detection result. The signal processing circuit 31 supplies a dc light beam to the optical current sensor 30, and calculates a current value based on the modulated optical signal returned by the optical current sensor 30. It should be noted that "dc light" as used herein refers to an output light beam obtained by driving the LED 315 with a dc current.
The relationship between the intensity P0 of light outputted from the signal processing circuit 31 to the optical current sensor 30, and the intensity P of the optical signal returned from the optical current sensor 30 to the signal processing circuit 31, can be derived from the following equation (1): EQU P=.alpha.P0(1+m*sin.omega.t) (1)
where m is the intensity modulation factor of optical current sensor 30 for an ac current signal Isin.omega.t, and .alpha. is a coefficient corresponding to the optical transmission loss of, for example, the optical current sensor 30, optical connectors 32, and optical fibers 33.
The signal processing circuit 31 obtains current value Isin .omega.t(.varies.m*sin.omega.t) by extracting only the (m*sin.omega.t) component from the intensity P derived from equation (1).
The optical-electrical conversion circuit 311 of the signal processing circuit 31 converts the optical signal from the optical current sensor 30 to an electrical signal. The subtraction circuit 312 then subtracts a specified value (K) from the converted electrical signal by the optical-electrical conversion circuit 311. The filter 313 extracts the dc component (V.alpha.) from the converted electrical signal by the optical-electrical conversion circuit 311. The LED driver 314 controls the drive current used for driving the LED 315. The LED 315 then outputs the light beam supplied to the optical current: sensor 30 according to the applied drive current.
The basic operation whereby the optical current sensor apparatus shown in FIG. 3 measures current is described next below.
When a predetermined initial drive current is applied in the signal processing circuit 31, the LED 315 emits a light beam. The light beam thus outputted from the signal processing circuit 31 is inputted through the optical fibers 33 to the optical current sensor 30, intensity modulated therein, and then returned again through the optical fibers 33 to the signal processing circuit 31. The optical-electrical conversion circuit 311 of the signal processing circuit 31 then converts the returned light intensity P to a current value V, and the subtraction circuit 312 further subtracts value K, equivalent to the dc component, from the current value V obtained by conversion to extract (m*sin.omega.t).
It should be noted that in the basic operation described above the value V.alpha. of the dc component contained in the current value V is not identical to the value K subtracted by the subtraction circuit 312 to account for the optical transmission loss of, for example, the optical current sensor 30, optical connectors 32, and optical fibers 33. This value V.alpha. also changes with, for example, aging of these and other components. As a result, an accurate current value Isin.omega.t(.varies.m*sin.omega.t) cannot be obtained by this simple subtraction operation. The optical current sensor apparatus shown in FIG. 3 therefore comprises a filter 313 to extract the dc component V.alpha. from the current value V obtained by conversion by the optical-electrical conversion circuit 311, and the LED driver 314 adjusts the LED 315 drive current so that the extracted dc component V.alpha. consistently matches a constant K.
The above-described method of adjusting the drive current is known as auto power control (APC). By applying APC, the input-output relationship of the signal processing circuit 31 shown in equation (1) can be restated using equation (2): EQU P =k(1+m*sin.omega.t) (2)
where k is a constant.
The current value V obtained by converting the intensity P derived from equation (2) by the optical-electrical conversion circuit 311 can therefore be derived from equation (3): EQU V =K(1+m*sin.omega.t) (3)
where K is a constant.
The current value V derived from equation (3) is then inputted to the subtraction circuit 312 where constant K is subtracted from V. As a result, the output Vout of the subtraction circuit 312 can be derived from equation (4). EQU Vout=V-K=K*m*sin.omega.t(.varies.m*sin.omega.t) (4)
The optical current sensor apparatus shown in FIG. 3 can thus obtain the current value Isin.omega.t(.varies.m*sin.omega.t) from the output passed by the subtraction circuit 312.
FIG. 4 is a block diagram of an exemplary optical current sensor apparatus according to another version of the related art. As shown in FIG. 4, this optical current sensor apparatus comprises an optical current sensor 40 and an optical current sensor signal processing circuit (hereinafter referred to as signal processing circuit) 41, which are connected by means of optical fibers 43 and optical connectors 42.
The signal processing circuit 41 comprises an optical-electrical conversion circuit 411, filter 412, subtraction circuit 413, divider circuit 414, LED driver 415, and LED 416.
The optical current sensor 40 contains a magneto-optical crystal (not shown in the figure), detects a change in (the field strength of) the current to be measured, and intensity modulates light according to the detection result. The signal processing circuit 41 supplies light to the optical current sensor 40, and calculates a current value based on the modulated optical signal returned by the optical current sensor 40.
The relationship between the intensity P0 of light outputted from the signal processing circuit 41 to the optical current sensor 40, and the intensity P of the optical signal returned from the optical current sensor 40 to the signal processing circuit 41, can be derived from the above-noted equation (1) where m is the intensity modulation factor of optical current sensor 40 for an ac current signal Isin.omega.t. The signal processing circuit 41 then extracts the (m*sin.omega.t) component from the intensity P value derived from equation (1), and obtains the current value Isin.omega.t(.varies.m*sin.omega.t). While this is substantially the same as in the optical current sensor apparatus shown in FIG. 3, the method used by the optical current sensor apparatus shown in FIG. 4 to extract the (m*sin.omega.t) component from intensity P differs as described below.
The LED driver 415 of this signal processing circuit 41 controls the drive current for driving the LED 416. It should be noted here that the LED driver 415 controls the output drive current to a predetermined constant level. The LED 416 thus outputs a light beam supplied to the optical current sensor 40 according to this applied constant drive current.
The optical-electrical conversion circuit then converts the optical signal from the optical current sensor 40 to an electrical signal. The filter 412 thus extracts the dc component V.alpha. from the electrical signal output by the optical-electrical conversion circuit 411. The subtraction circuit 413 subtracts the dc component V.alpha. extracted by the filter 412 from the electrical signal obtained by conversion by the optical-electrical conversion circuit 411. The divider circuit 414 then divides the remainder value passed from the subtraction circuit 413 by the dc component V.alpha. extracted by the filter 412.
The basic operation whereby the optical current sensor apparatus shown in FIG. 4 measures current is described next below. First, when a predetermined drive current is applied in the signal processing circuit 41, the LED 416 emits a light beam. The light beam thus outputted from the signal processing circuit 41 is inputted through the optical fibers 43 to the optical current sensor 40, intensity modulated therein, and then returned again through the optical fibers 43 to the signal processing circuit 41. The optical-electrical conversion circuit: 411 of the signal processing circuit 41 then converts the returned light intensity P to a current value V. The output V from the optical-electrical conversion circuit 411 can be derived from equation (5): EQU V=kVO(1+m*sin.omega.t) (5)
where kVO is a current value equivalent to .alpha.P0 when intensity P is converted to a current value.
The current value V derived from equation (5) is applied to the filter 412. The filter 412 then extracts current value V.alpha. from current value V where V.alpha. is the dc component shown in equation (6). EQU V.alpha.=kVO (6)
The output V from the optical-electrical conversion circuit 411 and the output V.alpha. from the filter 412 are thus applied to the subtraction circuit 413. The subtraction circuit 413 output and filter output V.alpha. are then applied to the divider circuit 414, which obtains output Vout as shown in equation (7). ##EQU1##
It will thus be obvious that the optical current sensor apparatus shown in FIG. 4 can therefore obtain the current value Isin.omega.t(.varies.m*sin.omega.t) from the output Vout passed by the divider circuit 414.
As will be known from the above, each of the optical current sensor apparatuses shown in FIG. 3 and FIG. 4 can remove measurement error introduced by intensity modulation other than that induced by the current being measured, and thus achieve accurate current measurement. Conventional optical current sensor apparatuses other than those as shown in FIGS. 3 and 4 and other types of conventional intensity modulated optical sensor apparatuses such as optical voltage sensor apparatuses can also measure desired physical quantities using a signal processing circuit constructed as described above.
A problem with such conventional apparatuses, however, is that the LED is continuously driven during the measurement process. Because much power is needed to drive the LED, a supply of several hundred milliwatts must be constantly supplied to meet the overall power requirements, of which the LED drive current is but one part, of the optical sensor signal processing circuit.
Conventional optical sensor apparatuses have been primarily installed as a means of monitoring transmission line loads and detecting fault information within the context of automating the power distribution network. Long-term installation in a single location is therefore common, and maintaining a constant power supply has not been a problem. In, for example, load monitoring applications, however, it has become necessary to monitor load changes at various points. This has led to the need for a portable optical sensor apparatus that can be easily used for monitoring where monitoring is required rather than at a single fixed location. Achieving such an optical sensor apparatus, however, requires low power consumption and the ability to operate the sensor apparatus using a battery or other power supply with limited capacity.
While power consumption can be reduced by using a pulse driven LED, for example, this is not simple for the following reasons. That is, to achieve an accurate measurement by means of an optical sensor apparatus, a process for optical-electrically converting an optical signal, and extracting a dc component from the converted electrical signal., is essential. As a result, all conventional optical sensor apparatuses have a filter for dc component extraction as in the exemplary apparatuses shown in FIGS. 3 and 4. However, such filter circuits typically have a large time constant, and anywhere from several seconds to several ten seconds is required for the filter to stabilize after a signal is input. Such conventional optical sensor apparatuses therefore cannot pulse drive the LED because the LED must be driven constantly so that the dc component extraction operation can be sustained continuously.